Electro-optic devices are frequently used in fiberoptic telecommunication systems to manipulate optical signals. Frequently, these electro-optic devices include at least one optical waveguide formed from and/or in an electro-optic material. When an electric field is generated in the electro-optic material, the refractive index of the optical waveguide(s) changes, and the optical signal propagating therethrough can be altered. Some examples of common electro-optic devices used in telecommunication systems include optical modulators, optical switches, optical couplers, etc.
One example of a commonly used electro-optic device is a Mach-Zehnder (MZ) optical modulator. Referring to FIG. 1A, there is shown an embodiment of a Mach-Zehnder optical modulator having an optical waveguide 20 formed in an electro-optic substrate 10. The optical waveguide 20 includes a first Y-branch 22, a first interferometer arm 24, a second interferometer arm 26, and a second Y-branch 28. An electrode structure, not shown in FIG. 1A, is provided near/adjacent the optical waveguide 20 for generating an electric field in one or both interferometer arms 24/26. For example according to one well known configuration, the electrode structure includes a signal electrode (also often referred to as a hot electrode) and two ground electrodes, which are configured to generate oppositely oriented electric fields in the first 24 and second 26 interferometer arms. Conventionally, the electrode structure is formed from a highly conductive metal such as gold (Au).
The exact position and design of the electrodes relative to the optical waveguide 20 is generally dependent on the substrate 10. If the substrate 10 is formed from X-cut lithium niobate (LiNbO3), a signal electrode 40 is typically positioned on top of the substrate substantially between the interferometer arms 24/26, while ground electrodes 42/44 are positioned on top of the substrate 10 outside of the interferometer arms 24/26, as shown in FIG. 1B. In contrast, if the substrate 10 is formed from Z-cut LiNbO3, the signal electrode 40 is typically positioned substantially directly above one interferometer arm 26, while the ground electrode 42 is positioned substantially directly above the other interferometer arm 24, as illustrated in FIG. 1C. In each case, the ground electrodes 42/44 are typically connected to ground, while the signal electrode 40 is connected to a high-frequency power source.
Referring to FIG. 1D, there is shown an embodiment of a Z-cut LiNbO3 modulator 100, wherein a signal electrode 140 is connected to a high-frequency power source 145 at one end and to a terminal resistor 146 at the other end, such that it functions as a traveling-wave electrode. In operation, an optical signal is input into the left side of the device 100 where it is transmitted through an optical waveguide 120 until it is split at a first Y-branch 122, and then propagates equally along two isolated paths corresponding to two interferometer arms 124/126. Simultaneously, an RF data signal from the high-frequency power source 145 is transmitted through an RF transmission line 147 (e.g., a coaxial cable) to the signal electrode 140, which functions as a microwave transmission line. As the modulation voltage is applied between the signal electrode 140 and ground electrodes 142 and 144, an electric field is generated in an underlying electro-optic substrate 110. As illustrated in FIG. 1E, the vertical electric field lines in the first 124 and second 126 interferometer arms are oppositely oriented, such that the light propagating in each of the arms 124 and 126 is complementarily phase shifted relative to one another in a push-pull fashion. In accordance with the electro-optic effect, the electric field changes the refractive index within the interferometer arms 124 and 126, such that the input optical signal experiences constructive or destructive interference at a second Y-branch 128. This interference produces an amplitude modulated optical signal that is output at the right side of the device 100, wherein the modulation corresponds to the original RF data signal.
Notably, since the Z-axis of a LiNbO3 crystal has the highest electro-optic coefficient, and the overlap with applied field is high, Z-cut LiNbO3 modulators exhibit a relatively high modulation efficiency. Unfortunately, Z-cut LiNbO3 modulators are also known to suffer more from charge build up problems, which may lead to temperature-induced bias drift and/or a DC-induced bias drift.
The term “temperature-induced bias drift” refers to drifting of the bias point of a modulator with changes in temperature. In LiNbO3, the temperature induced bias drift typically arises from the pyroelectric effect, which creates mobile charges when temperature fluctuations cause a mechanical stress in the substrate. The mobile charges can generate sufficiently strong electric fields to change the bias point of the electro-optic modulator. In addition, since the electric fields induced by the pyroelectric effect in Z-cut LiNbO3 are predominantly normal to the substrate, the mobile charges move toward the surface of the substrate, where the electrodes 140, 142, 144 are located. A bleed layer 160 is typically required near the surface of Z-cut LiNbO3 to dissipate the accumulated electric charge. Optionally, additional bleed layers (not shown) are used to dissipate charge at the sides or bottom of the substrate. In general, the bleed layer 160 will be formed from a semiconductive material having enough resistance to prevent the highly conductive electrodes 140/142/144 from shorting out.
The term “DC-induced bias drift” refers to drifting of the bias point of a modulator as a low frequency or DC voltage is applied to the modulator for extended periods of time. In general, low frequency or DC voltages are required to control the bias point of the modulator, which is the point about which the swing of the modulated RF signal is accomplished. For example, in the embodiment of FIG. 1D, the RF data signal 145 includes an RF component superimposed on a DC or low frequency component.
The DC-induced bias drifts are particularly problematic when the modulator 100 includes a buffer layer 150 as shown in FIG. 1E. The buffer layer 150 is disposed between a substrate 110 and the signal electrode 140. If the buffer layer 150 has little conductivity relative to the substrate 110, mobile charges within the substrate 110, which may be in the form of electrons, holes, or ions, counteract the effect of the applied voltage, establishing a positive DC-induced bias drift. In addition, impurities in the buffer layer 150, which is typically formed from a dielectric material such as silicon dioxide (SiO2), are believed to form additional mobile charge, which either counteracts the effect of the applied voltage, establishing a positive DC-induced bias drift, or enhances the applied bias voltage, establishing a negative DC-induced bias drift. The former is more common for undoped SiO2. The end result of the mobile charge accumulation in the buffer and substrate is that the bias voltage required to operate the electro-optic modulator varies over time.
The purpose of the buffer layer 150 is two-fold. First, the buffer layer 150 is used to prevent optical absorption of the optical signal by the overlying electrodes 140/142. Notably, this is more important for Z-cut embodiments, wherein the electrodes 140/142 lie directly over the interferometer arms 126/124. Secondly, the buffer layer 150 is used to speed up the propagation of the RF modulation signal so that the optical wave and the microwave propagate with nearly equal phase velocities, thus increasing the interaction length, and as a result, increasing modulation bandwidth and/or efficiency at high modulation frequencies.
Various solutions to prevent the DC-induced bias drift have been proposed. For example, for X-cut LiNbO3 modulators, it has been proposed to provide a separate low-frequency bias electrode structure 270, optically in series with an RF electrode structure 240, as illustrated in FIG. 2. A buffer layer 250 is provided below the RF electrode structure 240, to provide velocity matching, but is eliminated below the bias electrode structure 270, to reduce the DC drift. Conveniently, since the bias electrode structure 270 is deposited directly on the substrate, the required drive (bias) voltage is significantly reduced. Unfortunately, to accommodate both electrode structures, the length of the modulator has to be significantly increased. In addition, it is not ideal to locate the waveguides directly below the bias electrodes, because the highly conductive bias electrode material (e.g., Au) may introduce significant optical loss.
In Z-cut LiNbO3 modulators, the DC-induced bias drift has been reduced by modifying the buffer layer. For example, in U.S. Pat. Nos. 5,404,412 and 5,680,497, the effect of the buffer layer charging in optical modulators is reduced by doping the buffer layer, causing it to be more conductive. The added conductivity in essence shorts out the buffer layer, preventing the buffer layer from charging up and offsetting all of the applied voltage from the waveguides. Accordingly, a DC or slowly varying voltage applied to the signal electrode is able to control the bias point of the modulator over time.
Unfortunately, it can be difficult to quantitatively control the introduction of the doping elements with a high reproducibility at the small concentrations required. Furthermore, water may be absorbed by the conductive buffer layer, changing its properties. In addition, the required drive, or bias voltage may be relatively high because the generated electric field must pass through the conductive buffer layer, which may be quite thick for Z-cut configurations.
In US Patent Application Publication No. 2003/0133638, the DC-induced bias drift is reduced by implanting a SiO2 buffer layer with fluorine ions. The negative fluorine ions (F−) are believed to react with positive ions, such as lithium (Li+) from the substrate, to form stable compounds such as LiF. The reduction in the number of mobile Li+ ions then results in a reduction of the DC drift. Again, the required drive voltage may be relatively high, because the generated electric field must pass through the ion-implanted buffer layer, which may be quite thick for Z-cut configurations.
In US Patent Application Publication No. 2006/0023288 and U.S. Pat. No. 7,127,128, the DC-induced bias drift is reduced by providing a separate low-frequency bias electrode structure aligned with an overlying RF electrode structure. For example, consider the prior art X-cut embodiment illustrated in FIGS. 3A and 3B, wherein a dielectric buffer layer 350 is provided on a substrate 310 below an RF electrode structure 340/342/344, to provide velocity matching, but is eliminated below a bias electrode structure 370/372/374, to reduce the DC-induced bias drift. Unfortunately, using this structure with Z-cut electro-optical crystals results in increased optical loss, because in Z-cut crystals, the waveguides 324, 326 would have to be disposed directly under the bias electrodes 370, 372.
To reduce the optical loss, it has been proposed to dispose an additional buffer layer between the buried electrodes and the optical waveguide. For example, U.S. Pat. No. 7,844,149 discloses optical modulators having buried bias electrodes, in which a dielectric buffer underlayer is used to reduce optical loss.
US Patent Application Publication No. 2006/0023288 also describes buried-electrodes embodiments for Z-cut LiNbO3 modulators. Referring to FIGS. 4A and 4B, the Z-cut embodiments typically include two bias signal electrodes, each of which is split into two separate elongated segments. More specifically, each segment of each split bias electrode 470/476 is shifted laterally to an opposite side of a corresponding waveguide segment 426/424. Again, a dielectric buffer layer 450 is provided over a substrate 410 below a bleed layer 460 and an RF electrode structure 440, 442, 444, to provide velocity matching, but is eliminated below a bias electrode structure 470, 472, 474, 476, to reduce the DC-induced bias drift.
Two problems were discovered experimentally with buried bias electrodes disclosed in US Patent Application Publication No. 2006/0023288 and in U.S. Pat. No. 7,844,149. The first problem is that mechanical stress varies over temperature. The mechanical stress of the bias electrodes 470/476 causes an refractive index change due to the (1) elasto-optic effect and (2) piezoelectric effect followed by electro-optic effect, which cause a shift in the bias point over temperature. Unfortunately, it is difficult to match the mechanical stress created by the two identical bias signal electrodes over temperature. In particular, the split bias electrodes 470/476 shown in FIGS. 4A and 4B caused a large shift in the bias point with temperature. The second problem is that the optical loss is increased due to the small but significant conductivity of the bias electrodes 470/476 in close proximity to the waveguide.
Another well-known general problem of prior-art modulators relates to humidity resistance. The combination of high magnitude electric fields and high humidity often results in electro-migration corrosion. In addition, when a metal adhesion layer (e.g., Ti, Ti/W, Cr, etc) is used to promote adhesion between an RF electrode (e.g., Au) and an electro-optic substrate (e.g., LiNbO3), any moisture in direct contact with the multi-layer structure will serve as an electrolyte that induces galvanic corrosion.
Galvanic corrosion, which results from the difference in electrochemical potentials of dissimilar metals, can create a conductive deposit between the surface RF electrodes. The conductive deposit causes current leakage, short circuit, or peeling of the RF electrodes. Various schemes have been proposed to obviate the galvanic corrosion, and thus reduce the need for a hermetic package. For example, in U.S. Pat. No. 6,867,134 the adhesion layer is eliminated, whereas in US Patent Application Publication No. 2003/0062551 the adhesion layer is encapsulated. Alternatively, the adhesion layer can be made of a thin metal, such as nickel, which has a work function similar to gold. While these methods do suppress galvanic corrosion, electro-migration corrosion can still occur.
Electro-migration corrosion occurs when a large DC voltage is applied across closely spaced electrodes (e.g., gold RF electrodes) in the presence of water or at a high humidity level. Similar to galvanic corrosion, electro-migration corrosion negatively impacts the performance and reduces the service life of electro-optic devices. As a result, electro-optic devices are often coated as shown in U.S. Pat. No. 6,560,377 and/or sealed in hermetic packages.
The use of a coating is attractive because the modulator structure does not need to be modified dramatically to accommodate it. The coating has little impact on optical performance. A slight modification of electrode geometry, e.g., height, gap, might be required to compensate for a slight increase in the microwave refractive index caused by the dielectric properties of the coating. However, the coating can crack and/or delaminate due to a difference in thermal expansion coefficients of the coating materials and the electrodes being coated. Cracked and/or delaminated coating allows the moisture to penetrate the device, which causes corrosion and subsequent device failure.
In US Patent Application Publication No. 2006/0023288, the humidity tolerance is increased in various ways. In some embodiments, the large DC voltage is applied to bias electrodes that are disposed beneath a buffer layer, whereas in other embodiments the large DC voltage is applied to bias electrodes that are disposed below the substrate. Since these buried bias electrodes are protected from humidity, electro-migration corrosion of the buried bias electrodes is reduced.
The prior-art solutions to DC drift and humidity resistance problems rely on complex bias electrode and/or complex layer structures. In many prior-art modulators, using buried bias electrodes inevitably resulted in an increased optical loss. It would be advantageous to provide an electro-optical device with reduced DC drift and moisture sensitivity without the excessive layer complexity and/or optical loss penalty. Accordingly, it is a goal of the present invention to provide such a device.